1. Field of the Invention
The present invention relates to an electronic circuit device that includes a heat-generating element such as a power transistor mounted on a circuit board.
2. Description of Related Art
An electronic circuit often includes a heat-generating element such as a power transistor. If such a heat-generating element mounted on a circuit board heats up a circuit beyond a permissible temperature, the circuit may malfunction or the circuit may be damaged. Therefore, it is very important to properly dissipate heat generated in the electronic circuit.
For properly dissipating heat generated in an electronic circuit, it is required to provide a sufficient heat-dissipating space around the heat-generating element mounted on a circuit board. However, providing a large space for heat dissipation is contradictory to a down-sizing requirement. To cope with this problem, JP-A-7-79053 proposes a structure for dissipating heat generated in a power element mounted on a circuit board. The power element is mounted on an upper surface of the circuit board, and heat generated in the power element is transferred to an inner layer provided in the circuit board through a heat-conductive path formed underneath the power element. In this structure, however, the heat-conductive path is made only from the upper surface to a center portion of the circuit board. In other words, the heat-conductive path does not reach a lower surface of the circuit board. Therefore, the heat is not sufficiently dissipated from the circuit board to an atmospheric space. In addition, it is required to make a relatively large heat-conductive path underneath the power element, and therefore the structure becomes complex and the manufacturing cost becomes high.